1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier and a printer, which has a function of forming an image on a recording medium such as a sheet.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a copier and a laser beam printer, for example, an image formation is performed in the following manner. An electrophotographic photosensitive member (photosensitive member) is irradiated with light corresponding to image data to form an electrostatic image (latent image) on the photosensitive drum. A toner as a developer which is a recording material is supplied from a developing device to the electrostatic image to develop the electrostatic image into a toner image. The toner image is then transferred from the photosensitive member onto a recording medium such as a recording sheet by a transfer device. The toner image is fixed to the recording medium by a fixing device to form a recording image finally.
A developer containing portion is connected to the developing device, and the developer contained in the developer containing portion is conveyed into the developing device by a developer conveying and agitating member, and then the developer is consumed by forming the toner image.
(Detection of Remaining Amount of Developer)
In the image forming apparatus described above, when the apparatus has run out of developer, a user replaces a cartridge with a new one or replenishes the toner to enable the image formation. Therefore, such an image forming apparatus may include a unit which detects the consumption of the developer to inform the user of the consumption, i.e., a developer amount detecting device.
As the developer amount detecting device, there is known a capacitance measuring system that includes at least a pair of input side and output side electrodes and detects the developer amount by measuring a capacitance between the electrodes. As one type of the system, a plate antenna system has been known.
In the plate antenna system, for example, if a developing method of applying an alternate-current bias to a developer carrying member of a developing device is employed, a metal plate serving as an electrode is disposed at a position opposite to the developer carrying member.
The developer amount is then detected by using a capacitance between the metal plate and the developer carrying member based on a current flowing through the metal plate serving as an electrode, which is changed depending on the amount of the developer such as an insulating toner (Japanese Patent Application Laid-Open No. 2003-255688).
Specifically, when a space between the metal plate and the developer carrying member or between two metal plates is filled with the developer, the capacitance is increased, and as the amount of developer decreases, a ratio of the air occupying the space between the metal plate and the developer carrying member or between the two metal plates is increased, and thus the capacitance decreases. Therefore, if a relationship between the developer amount and the capacitance between the metal plate and the developer carrying member or between the two metal plates is obtained in advance, a level of the developer amount can be detected by measuring the capacitance.
However, the conventional technology described above possibly has the following problem.
In the image forming apparatus, a peak-to-peak voltage or a frequency of the developing bias may be changed to obtain a stable image. The remaining amount of the toner is calculated based on a capacitance which is obtained from a value of current flowing through a toner remaining amount detecting member at the time of applying a developing alternate-current voltage. Therefore, the change of the peak-to-peak voltage or the frequency which are parameters of the current value changes the value of current so that there is a possibility that an output value of the detection of the level of the toner remaining amount is changed.
Up to now, there are a technique in which only in a process other than an image forming process, a constant developing bias is applied to detect the toner remaining amount and a technique in which the output value is corrected by control. However, in order to detect the toner remaining amount with more accuracy, it has been demanded to achieve an improvement of accuracy.